About to cross-reference this, but I think dewaarimasen might be separated into three separate words, "de wa arimasen". This link shows it used that way (under polite forms of the word keiyodoshi, neg. polite past), but I'm about to check for more references.
http://en.wikipedia.org/wiki/Japanese_adjectivesThis may just be a different way to say it (dewaarimasen might be a contraction), but I just wanted to mention it in case something's wrong.
Edit: New link -
http://en.wikipedia.org/wiki/Japanese_grammarUse Ctrl + F to search for "De wa arimasen", or just scroll down to the second chart (big chart) under adjectives (or just press "4.3 Adjectives" under contents, then scroll down to the second chart).
Edit2: Also noticed that alot of sites use "dewa arimasen" also. Haven't come across "Dewaarimasen" much though.